Added a test case for bug #9210.
sql_select.cc:
  Fixed bug #9210.
  The function calc_group_buffer did not cover the case
  when the GROUP BY expression was decimal.
  Slightly optimized the other code.


sql/sql_select.cc:
  Fixed bug #9210.
  The function calc_group_buffer did not cover the case
  when the GROUP by expression was decimal.
  Slightly optimized the other code.
mysql-test/t/func_group.test:
  Added a test case for bug #9210.